Variable impedance



Sept. 2, 1941. HAMMOND 2,254,366

VARI'ABLE IMPEDANCE Filed July'18 1936 l 2 Sheets-Sheet 1 Jae 5%??- L. HAMMOND VARIABLE IMPEDANCE Filed July 18, 1936 Sept. 2 1941.

2 Shets-Sheet 2 Patented Sept. 2, 1941 UNITED STATE s PATENT OFFICE VARIABLE IMPEDANCE Laurens Hammond, Chicago, Ill.

Application July 18, 1936, Serial No. 91,284

Claims. (01. 201-48) The present invention relates generally to variable impedance devices, and more particularly to devices of this kind usable in electrical musical instruments, radio receivers, and similar electrical apparatus.

The object of the invention is to provide an improved variable impedance in which the variations in impedance are caused by successive connection in parallel of incremental impedances.

A further object is to provide an improved form of switch for successfully making or breaking a plurality of contacts.

A further object is to provide an improved variable impedance for electrical apparatus in which transient frequencies due to the making and breaking. of contacts are substantially eliminated.

A further object is to provide an improved variable impedance which is simple in construction, is very durable, and which may be manufactured at relatively low cost.

Other objects will appear from the following description, reference being had to the accompanying drawings, in which:

Fig. 1 is a fragmentary rear elevational view showing the variable impedance device of my invention as applied to an electric organ;

Fig. 2 is a. sectional view taken, on the line 2-2 of Fig. 1;

Fig- 3 is a vertical sectional view taken on the 11118 3-3 of Fig. 2;

Fig. 4 is a horizontal sectional view taken on the line 4--4 of Fig. 3;

Fig. 5 is a vertical sectional view taken on the line 5-5 of Fig. 4; and

Fig. 6 is an exemplary wiring diagram illustrating how the variable impedance may be connected in the output circuit of an electric organ In many types of electrical musical instruments, it is desirable to have in the output circuit a variable impedance by means of which the volume of the sound may be controlled or by which some other characteristic of the circuit may be adjusted or regulated. For ex-' ample, in the electric organ of my invention shown in my prior Patent No. 1,956,350, granted April 24, 1934, a swell pedal is employed to control the volume of the sound produced by the instrument.

In said patent, the swell pedal is illustrated as operating a potentiometer or variable resistance of the usual type in which a movable contact member slides over a resistance winding, making contact with successive turns thereof. In playing the electric organ, the organist moves the swell pedal very frequently, with the result that there is considerable wear on the sliding contact member. Furthermore, a sliding contact member has a tendency to introduce transients into the control circuit because of minor irregularities in the contact resistance as the sliding contact is shifted along the resistance wire. To obviate these disadvantages of the sliding contact type of potentiometer pedances, I have provided an improved form of variable impedance for use in electric organs and other electric instruments and apparatus in which the impedance is varied by successively connecting incremental impedances in parallel.

In Fig. 1, the variable impedance is shown as enclosed in a casing ill of sheet metal, the ends' l2 of which form supports therefor and rest upon a, shelf I4 forming part of the instrument. The swell pedal l6, by which the variable impedance is operated, is hinged to a suitable support l8 and has a crank arm 20 rigidly secured thereto. The oscillatory stroke of the swell pedal I 6 is limited in one direction by the engagement of a stop 22 with the support l8, and in the other direction by an adjustable stop 24 which is threaded in the support I8 and may be screwed inwardly or outwardly as desired to limit the maximum volume of sound produced by the instrument. The free end of the crank arm 20 is pivotally connected to a'rod 26 which I is suitably guided in a bracket 28 secured to the top of the casing lil and which is threaded at its upper end.

A plate 30 is clamped in adjusted position to the upper end of the rod 26, this plate extending sidewardly and projecting into a notch 32 in a switch actuating cam member 34 made of insulating material. It will be apparent that oscillations of the swell pedal it will cause vertical reciprocation of the 'cam member 34 and that when the swell pedal is in the position shown in Fig. 2 the cam member 34 will be raised to the upper limit of its stroke.

Within the casing H) is a supporting frame comprising a top plate 36, to which are riveted a pair of spacing studs 38, 40 and a lower plate 42 which is secured against the shouldered ends of the spacing studs 38, 40 by nuts 44 and lock washers 4B- The plates 36 and 42 are provided with slots to receive the ends of the switch finger supports 48, 49, and 5|. Impedance supand of similar variable im-- porting bars 52, 54 are similarly mounted between plates 36 and 42 by having their ends 56, 51 of reduced width project through complementary slots formed in the plates 36, 42. The switch finger supports 48, 49, 58 and 5| are of insulating material, preferably of fabric reenforced sheets of a phenolic condensation prodnot, while the impedance element supporting bars may be made of any suitable conducting sheet metal.

Sheets 58, 59 of insulating material are riveted to the plates 36, 42 respectively, and are provided with slots 58a and 59a respectively to receive the ends 6|] and 6| of two pairs of bus bars 62, 63, 64, 65. Each of these bus bars is preferably made of a suitable nickel alloy and has welded to its outwardly facing edge a wire 66 which is preferably made of a platinum-iridium alloy or a palladium alloy so as not to be subject to oxidation or corrosion.

A plurality of switch fingers 68 are secured to their supports 48, 49, 50 and 5| by rivets 10. These switch fingers are preferably made of thin strips of phosphor bronze or similar elastic metal. In the embodiment illustrated, there are four banks of switch fingers, each bank comprising eight switch fingers. At their free ends, the switch fingers 68 have small strips II of a suitable nickel alloy spot welded thereto, and these strips ll have short wires 12 welded thereto for engagement with the contact wires 66 which are welded to the bus bars 62, 63, 64 and 55. The outer ends of the strips II of two of the rows of switch fingers are alternately interposed to overlie the edges of the cam member 34. The wires 12 are preferably made of a suitable alloy of a metal of the platinum group, such as platinum, palladium, or iridium.

It will be noted that the cam member 34 has camming surfaces 14 and I6 formed on its opposite edges, these surfaces being spaced with respect to the longitudinal dimension of the cam member 34 so that the strips II of the switch fingers will be engaged by these cam surfaces sequentially, that is, the cam surfaces 14 and 16 will, upon downward movement of the cam member 34, alternately engage the switch fingers so that the contacts will be broken seriatim.

Each of these switch fingers 68 has an apertured rearwardly extending ear 18 formed integrally therewith to which the terminal of a resistor 80 is connected. The other terminal wire of the resistor projects through one of the rows of apertures formed in the rmistor supporting bars 52. The connections between the terminals of the resistors and the switch fingers, as well as the similar connection betweenthe other terminals of the resistors and the resistor supporting bars 52, are preferably soldered. The narrow lower ends 51 of the resistor supports 52 form soldering lugs for connecting the variable impedances to the circuit to be controlled, while the narrow ends 6| of the bus bars 62 similarly form soldering lugs for connection to the other portion of the circuit to be controlled. The cam member 34 is guided for longitudinal movement in suitable slots 36a and 42a formed in the upper frame plate 36 and lower frame plate 42 respectively. The switch assembly is secured in position within the casing ID by means of screws 82 which are threaded in the studs 38, 48. V V

Fig. 6 is an illustrative wiringdiag'rainto show a circuit in which the variable impedance above described may be advantageously used. The circuit represents, in general, a portion of the outw put circuit of the electric organ described in my aforesaid patent. The circuit comprises a transformer having a primary 84, and a secondary 86, the primary bein supplied with the signal pulsations through conductors 88. The secondary 66 of the transformers has its ends connected to the amplifying circuits by wires 90, 92. To control the volume of the sound produced by the organ, the impedance is bridged across the output conductors 90, 92, this impedance being illustrated as comprising a fixed condenser 94 and the variable impedance device above described, the variable impedance being connected in series with the capacitance 94.

The bus bars 62, 63, 64, 65 are represented in Fig. 6 by the resistor supporting bars 52, 54 and by wires given corresponding reference characters. The resistance elements are shown as being capable of being connected between the conduction bars 52 and bus bars 63 and 65 respectively as well as between the resistor support 54 and bus bars 62 and 64 respectively. In this figure, however, an inductance 96 is illustrated as replacing one of the resistors 80 and a condenser 98 as replacing another one of these resistors. Depending upon the type of control desired, the impedances 80 may be resistances, condensers, inductances, or any desired combination of these electrical impedances.

The variable impedance in the form disclosed herein is operated by swinging the swell pedal 16 which of course results in vertical reciprocatory movement of the cam member 34. As the cam member moves downwardly, its cam surface 16 will first engage the uppermost of the switch finger strips H and raise its contact wire 72 from contact with the contact wires 60 carried by the bus bar 62. Thereafter, upon continued downward movement of the cam member 34, its cam surface 14 will similarly open the uppermost switch which is in engagement with the bus bar 62. Thereafter, switches will be successively raised from contact with the contact wires of bus bars 63, 65, 62, 64, etc. until all of the switches have been opened. Upon upward movement of the member 34, the switches will close in the reverse order.

All of the switch fingers 68 are resilient and are normally bowed slightly as indicated in Fig. 4 so as to hold their contact wires 12 firmly in contact with the contact wires 66 attached to the bus bars. Since the sprin fingers are released relatively slowly by the cam surfaces 14 and 76 of the cam member 34 and the voltage and amperage of the currents carried by these switch contacts is very minute, there will be very little mechanical wear and no erosion or pitting of the contacts. While the impedances utilized when the present invention is used in conjunction with an electric organ are preferably resistors, it may be desirable in certain applications of the present invention to substitute inductances or capacitances for the resistances, as indicated in the wiring diagram of Fig. 6. By proper selection of the types and values of the impedances used, the movement of the cam member 34 may be utilized to impart to the circuit controlled any desired variable characteristic. For example, during the first portion of the downward stroke of the cam member 34, a number of capacitive impedances might be successively disconnected,

from the circuit, upon further movement of the cam member 34 a non-inductive resistance might be removed from the circuit, and upon the final portion of the downward stroke of the cam member, inductive impedances might be successively removed from the circuit.

In using the above described variable impedance device, it has been found that the volume of sound produced may be very easily controlled without introducing any delta transients or other unwanted noise currents. The variable impedances, although used a great deal during the playing of the instrument, have been found to be very durable, and from accelerated endurance tests, have been found to outlast the other parts of the organ, whereas similar tests had shown that the customary variable resistance having a sliding contact would have a very-short useful life.

While I have shown the variable impedance as applied particularly to an electric organ, it will be understood that it may be used in other electrical musical instruments and apparatus, and that many variations in the form of the invention may be made without departing from the'principles thereof. I therefore desire to include within the scope of the following claims all similar constructions wherein substantially the same result may be obtained in substantially the same way.

What I claim and desire to secure by Letters Patent is:

1. In a variable impedance device, the combination of a pair of frame plates, a plurality of switch finger supports of insulating material extending between said plates and held in position thereby, a plurality of bus bars extending between said plates in a direction parallel to said switch finger supports, said bus bars being insulated from said frame plates, a plurality of resilient switch fingers secured to each of said switch finger supports and each having a contact engaging one of said bus bars, an impedance element connected to each of said switch fingers, a cam member of insulating material guided in said frame plates for reciprocation in a direction parallel to said bus bars and perpendicular to said switch fingers, said switch fingers having portions lying in the path of movement of the cam surfaces of said cam member, and means for reciprocating said cam member, thereby to cause engagement of the cam surfaces thereof with said switch fingers successively to raiseithe latter from contact with said bus bars.

2. In a variable impedance device, a pair of spaced frame plates having similar slots Iormed therein, a plurality of supporting elements of insulating material having portions of reduced transverse dimensions projecting through said slots and held between said plates, a plurality of bus bars extending between said plates and secured thereto but insulated therefrom, a plurality of rows of switch fingers secured to said supports and having their free end portions normally resiliently engaging said bus bars, impedance elements respectively connected to said switch fingers, and a reciprocable member of insulating material having a camming surface for engaging the free end portions of said switch fingers and operable successively to move the fingers from contact with said bus bars.

3. In a variable electrical impedance device, the combination of a plurality of bus bars, a plurality of rows of switch fingers having con- .tacting portions for engagement with said bus bars, there being one row oi! switch fingers for each of said bus bars and each switch finger being insulated from the other, an impedance element connected to each of said switch fingers, and a single actuator of insulating material operable upon movement thereof in one direction sequentially to raise said switch fingers from contact with their respective bus bars.

4. In a variable electrical impedance device, the combination of a first conductor and a second conductor, a row of switch fingers having contact portions adjacent their free ends for making electrical contact with said first conductor, a second row of switch fingers extending in a direction substantially opposite to that of said first row or switch fingers and having contact portions at their free ends interdigitated with the contact portions of said first row of switch fingers and engageable with said second conductor, an actuator of insulating material movable transversely of the interdigitated portions of both of said rows or switch fingers and having a cam portion for successively engaging said interdigitated portions and moving the adjacent contact portions away from their respective conductors, and impedance elements connected respectively to said switch fingers for connection with their associated conductor upon operation of said actuator.

5. In a variable electrical impedance device, the combination of two pairs or geometrically parallel conductors, a row of switch fingers for each of said conductors, the rows of switch fingers for each or the pairs of conductors having overlapping portions, said switch fingers being resiliently pressed against their associated conductors, a single actuator of insulating material having a pair of camming surfaces engageable respectively with the overlapping portions 0! the pairs of rows or said switchfingers to operate them successively, and an impedance element connected to each of said switch fingers for connection in circuit with its associated conductor upon operation of said actuator.

LAURENS HAMMOND. 

